[unreadable] [unreadable] Regulating the dynamic behavior of microtubules is essential for many cellular functions. Several factors have been identified that play central roles in modulating microtubule dynamics in vivo. In Xenopus egg extracts, the protein XKCM1 provides the dominant microtubule destabilizing activity. XMAP215 stabilizes microtubules by opposing the function of XKCM1. However, the mechanism by which XMAP215 antagonizes XKCM1 activity remains unclear. I will establish several assays to directly examine the opposing relationship between these two proteins. I will set-up a system using total internal reflective microscopy (TIRF) to investigate whether XMAP215 inhibits the binding of XKCM1 to the plus-ends of microtubules, or its ability to promote microtubule depolymerization. Using electron microscopy I will examine whether these proteins influence the structure of the microtubule lattice. And I will undertake a functional analysis of individual domains of XMAP215 to determine which regions of the protein are important for inhibiting XKCM1 function. Using this combination of techniques I will characterize the antagonistic relationship between XMAP215 and XKCM1, and begin to elucidate how the interplay between these proteins affects microtubule behavior. [unreadable] [unreadable] [unreadable]